Fiber reinforced polymers (FRPs) are widely used to create structural elements and parts used in aerospace, automotive, watercraft, sporting goods, and civil/structural engineering applications among others. FRPs are strong, light-weight materials with high strength-to-weight ratios. FRPs typically are formed by combining fibers and an uncured binding thermosetting polymer or resin and then curing the binding polymer or by combining fibers with a thermoplastic resin by melt infusion under heat and pressure. Some non-limiting examples of FRPs include carbon fiber reinforced polymers (CFRPs) and glass fiber reinforced polymers.
The wide use of FRPs has led to waste disposal issues and a demand for FRP recycling. In addition, certain reinforcing materials such as carbon fibers are expensive making their recovery and reuse economically desirable.
Three general recycling methods that disrupt the polymer matrix have been used to recover free fibers from FRPs: mechanical, thermal, and chemical recycling. Mechanical FRP recycling uses mechanical methods such as grinding to convert large FRP pieces into small pieces and particles, ultimately resulting in resin-rich powders and very small fibers. A problem with mechanical recycling is that the polymer cannot be separated from the fibers and fiber length cannot be controlled. As a result, recovered fibers are of little value.
Thermal FRP recycling typically entails pyrolyzing FRPs in a controlled oxygen environment at very high temperatures to combust the polymer and leave recoverable fibers. Because pyrolysis occurs at very high temperatures, recovered fibers are often weakened and charred during the recycling process.
Chemical recycling converts the polymer portion of recycled FRPs into oligomers or monomers through depolymerizing the polymer matrix via a process requiring supercritical or near-supercritical pressure, through depolymerization under heat and pressure with an alkaline catalyst, or through the use of ionic liquids at atmospheric pressure. Alkaline catalysis recovery is slow and often results in incomplete removal of resin components. At supercritical pressures, chemical recycling is expensive and dangerous. Additionally, ionic liquids that are capable of depolymerizing FRPs are expensive and susceptible to degradation through oxidation and ionization.
The volume of FRP's entering the waste stream from composite material disposal is expected to grow. In addition, the value of the FRP fiber components such as carbon fibers can be quite high if they can be recovered in useful amounts and lengths. Therefore, there is a need for environmentally friendly and inexpensive FRP recycling processes that can recover salable fibers.